1. Field of the Invention
This invention relates to optical inspection systems, and more specifically, to an optical system incorporating a resonator to produce an enhanced sensitivity to defects with significant height and a filtering of surface roughness.
2. Description of the Related Art
Precision surfaces are required in components used in many applications today. Storage media for use in both for optical and magnetic storage systems require surfaces that are free of defects having significant height above the mean surface of the media. If a defect sufficiently high collides with the storage system head assembly, the storage system may be destroyed or damaged.
In semiconductor manufacturing, semiconductor material is etched from and deposited on a silicon wafer. Yield of semiconductors is dependent on defect-free regions on the surface of the wafers. Defects of significant height may cause shorting between layers within an integrated circuit and may also reduce reliability of integrated circuits that appear otherwise functional after manufacture.
In the manufacturing process, it is necessary to inspect media, wafers and other surfaces to determine whether or not they are manufactured to the tolerances demanded by functional requirements and to make necessary adjustments in the manufacturing process to avoid manufacturing defective components.
Near field inspection systems may be used to resolve small defects, but since the near-field is confined to a relatively small height above the surface under inspection, it must scan the surface very slowly to avoid collision with the surface and if defects of significant height exist, the near-field probe may be damaged by collision with a defect.
Present far-field inspection systems use interferometric techniques to determine surface height by reflecting and measuring an optical beam off the surface under inspection. However, standard far-field inspection systems are unsuitable for detecting small profile defects having significant height to above the average surface.
Any optical inspection system has resolution limits. Within a resolution cell dictated by the resolution limits, the reflected field will be averaged from all points within the aperture of the resolution cell. The resolution cell limits are both angular and linear and are affected by surface characteristics in that very small surface features disperse a reflected field over a wide angle. Small sub-wavelength defects and surface variations approach point source behavior, which will cause reflected energy to be dispersed throughout a half-plane (180 degree solid angle) above a surface under inspection.
Within the resolution cell of an optical inspection system, the received reflected energy is averaged. A defect that is significantly smaller than the resolution cell and having a height or depth that is slightly greater than the acceptable surface roughness will produce an optical signal that is indistinguishable in the presence of the xe2x80x9cspeckle noisexe2x80x9d produced by the surface roughness.
Due to the angular spectrum of small defect reflections (reflecting into approximately the entire half-plane) and the resulting interference with surface roughness variations within the resolution cell being measured, the sensitivity of existing far-field inspection systems to small (sub-wavelength) defects is further reduced.
In essence, a wide-profile deviation of nominal depth or height that is acceptable, may produce the same or greater inspection signature as a very small-profile defect of unacceptable height. Therefore existing far-field optical systems cannot discriminate between small defects and normal roughness variations. Thus, existing far-field optical inspection systems are unsuitable for inspecting surfaces for small defects. Since the profile of a defect that may cause damage to a media storage device or shorting in an integrated circuit wafer may be very small, existing far-field inspection systems are unsuitable for detecting the above-mentioned defects.
Therefore, it would be desirable to provide a far-field inspection method and apparatus having an enhanced sensitivity to defect height. It would further be desirable to provide an inspection method and apparatus having a filtering characteristic for reducing the impact of surface roughness on inspection sensitivity. It would further be desirable to provide a far-field inspection method and apparatus that reduce the angular spectrum of reflections from a small defect to improve discrimination between small defects and surface roughness.
The foregoing objectives are achieved in an optical inspection method and apparatus having an enhanced height sensitivity region and roughness filtering. The inspection apparatus includes an optical illumination system for producing a beam for illuminating a surface under inspection, a detector for detecting intensity of light reflected from the surface under inspection, and a partially reflective surface positioned between the illumination subsystem and the surface for forming an optical resonator between the partially reflective surface and the surface under inspection. The resonator improves the sensitivity of the detector to reflections from defects having a height exceeding a predetermined height. The sensitivity is increased due to multiple reflections within the resonator. The resonator may be tuned so that the sensitivity of the inspection system is decreased for surface variations below a predetermined value and increased for variations above the predetermined value, so that filtering of acceptable roughness variation is achieved. The resonator also reduces the angular spectrum of reflections from small defects.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following, more particular, description of the preferred embodiment of the invention, as illustrated in the accompanying drawings.